1. Field of the Invention
The present invention relates to a lithium-titanium complex oxide suitable as an electrode material for lithium ion secondary battery, as well as a manufacturing method thereof.
2. Description of the Related Art
Development of lithium ion secondary batteries as high-capacity energy devices has been active in recent years, and lithium ion secondary batteries are beginning to be utilized in consumer equipment, industrial machinery, automobiles and various other fields. Characteristics required of lithium ion secondary batteries include high energy density, high power density and other characteristics that support high capacity and allow for quick charge/discharge. On the other hand, incidents of fire involving a lithium ion secondary battery have been reported and the market is demanding greater safety of lithium ion secondary batteries. In particular, lithium ion secondary batteries used in onboard applications, medical applications, etc., directly affect human life in case of accidents and require even greater safety. Safety is also required of materials used for lithium ion secondary batteries, where, specifically, the market is demanding materials that demonstrate stable charge/discharge behaviors and will not burst or ignite even in unforeseen accidents.
Lithium titanates are expressed, for example, by Li4Ti5O12, Li4/3Ti5/3O4 or Li[Li1/6Ti5/6]2O4, and have a spinel crystalline structure. The aforementioned lithium titanate changes to a rock-salt crystalline structure as lithium ions are inserted during charge, and changes back to a spinel crystalline structure again as lithium ions dissociate. Lithium titanate undergoes far less change in its lattice volume due to charge/discharge compared to carbon materials that are conventional materials for negative electrodes, and generates little heat even when shorted to the positive electrode, thereby preventing fire accidents and ensuring high safety. Lithium-titanium complex oxides whose main constituent is lithium titanate and to which trace constituents have been added as necessary, are beginning to be adopted by lithium ion secondary battery products that are designed with specific focus on safety.
Tap density of powder, which is traditionally evaluated as one general powder property required of battery materials including lithium-titanium complex oxides, is an important factor that affects handling of powder and becomes particularly useful when the sizes of primary particles constituting the powder are relatively large in a range of about 5 μm to several tens of μm or when an electrode coating film is formed directly from the granulated powder. On the other hand, powder properties of lithium ion secondary battery materials are drawing renewed attention in recent years in order to support the high-performance needs of lithium ion secondary batteries, and as part of this trend, attempts are being made to reduce the primary particle size of powder. This is an important factor that affects quick charge/discharge (rate characteristics) as the smaller the particle size, the smoother the insertion/dissociation reactions of lithium ions become and good characteristics are achieved as a result.
Methods to make the particles constituting the powder finer include the method to use the liquid phase method to make the primary particles themselves fine (build-up method) as described in Patent Literature 1, and the method to crush the primary particles after giving them a relatively rough heat treatment to make them finer (breakdown method) as described in Example 1 of Patent Literature 2. There is also a method, which is not the liquid phase method, whereby a very fine titanium compound is used as the material and mixed with a lithium compound, and then the mixture is heat-treated at low temperature to manufacture fine lithium titanate particles. Patent Literature 3 touches on the particle size distribution measured by laser diffraction and reports that the particle size distribution affects on rate characteristics.